Field of the Invention
The present invention relates to an exposure apparatus, an exposure method, and a method of manufacturing an article.
Description of the Related Art
An exposure apparatus which projects and transfers the pattern of a reticle (mask) onto a substrate by a projection optical system is employed to manufacture a device such as a semiconductor device or a liquid crystal device. One of important performances of such an exposure apparatus is the overlay accuracy of respective patterns transferred onto the substrate upon a plurality of processes. The imaging characteristics (for example, the focus, magnification, distortion, astigmatism, and wavefront aberration) of the projection optical system are an important factor which influences the overlay accuracy. In recent years, the pattern of a semiconductor device (for example, a VLSI) has a strong tendency to miniaturize, and a demand for an improvement in overlay accuracy is soaring to keep up with this tendency.
In an exposure apparatus, when exposure is repeated, variations in imaging characteristic of a projection optical system occur as the projection optical system is heated upon partially absorbing the energy of exposure light or dissipating the absorbed heat. Such variations in imaging characteristic of the projection optical system are called a heat aberration or an exposure aberration, and lead to degradation in overlay accuracy. Under the circumstance, Japanese Patent Publication No. 63-16725 and Japanese Patent Laid-Open Nos. 11-317354 and 63-58349 propose techniques for compensating for variations in imaging characteristic of a projection optical system (to be referred to as an “exposure aberration” hereinafter) as exposure light is guided to the projection optical system.
Japanese Patent Publication No. 63-16725 discloses a technique of calculating an exposure aberration in accordance with a model equation (to be also simply referred to as a model hereinafter) which uses, for example, the amount of exposure, the exposure time, or the non-exposure time as variables, and adjusting (correcting) the imaging characteristics of the projection optical system based on the calculation result. This model equation has a coefficient unique to the projection optical system for each imaging characteristic, and the coefficient changes depending on the illumination shape (effective light source shape) used to illuminate a reticle, or the pattern shape of the reticle. The model equation (coefficient) depends on the effective light source shape because the position through which exposure light passes in the projection optical system changes depending on the effective light source shape formed by an illumination optical system. On the other hand, the model equation depends on the pattern shape of the reticle because the diffraction direction of light (diffracted light) diffracted by the pattern changes depending on the pattern pitch, so the position through which the diffracted light passes in the projection optical system and its periphery also changes.
Also, Japanese Patent Laid-Open Nos. 11-317354 and 63-58349 propose techniques of obtaining a model equation (coefficient) for each effective light source shape or each pattern shape of a reticle. Japanese Patent Laid-Open No. 11-317354 discloses a method of obtaining, in advance, a correlation table between the effective light source shape and the model equation coefficient before an exposure apparatus is used. Japanese Patent Laid-Open No. 63-58349 discloses a method of performing exposure (actual exposure) while correcting the exposure aberration using the above-mentioned model equation, and detecting a correction error of the exposure aberration to correct (fine correction) the model equation coefficient.
Unfortunately, it is difficult for the conventional techniques to efficiently obtain a model equation that can be used to calculate the exposure aberration in actual exposure, leading to considerable degradation in performance (for example, overlay accuracy) of the exposure apparatus due to the exposure aberration. In Japanese Patent Publication No. 63-16725 and Japanese Patent Laid-Open No. 11-317354, the pattern shape of the reticle is not taken into consideration in obtaining a model equation (coefficient), so the exposure aberration in actual exposure cannot be obtained with high accuracy even when a model equation is used. On the other hand, in Japanese Patent Laid-Open No. 63-58349, a model equation can be obtained in consideration of the effective light source shape and the pattern shape of the reticle. However, because it takes a long time to obtain such a model equation, it is impractical to obtain model equations for all effective light source shapes and all pattern shapes of reticles used in the exposure apparatus.